1. Field of the Invention
This invention relates to a document authentication method which uses barcodes to encode content of the document, and in particular, it relates to such a document authentication method in which hierarchical barcode stamps are used as a background pattern to reduce the impact of alteration of barcodes.
2. Description of Related Art
Barcode is a form of machine-readable symbology for encoding data, and has been widely introduced in a variety of application fields. Two-dimensional barcode (2d barcode) is one mode of such symbology. It can be used to encode text, numbers, images, and binary data streams in general, and has been used in identification cards, shipping labels, certificates and other documents, etc. Examples of widely used 2d barcode standards include PDF417 standard and QR Code®, and software and hardware products have been available to print and read such 2d barcodes.
Original digital documents, which may include text, graphics, images, etc., are often printed, and the printed hard copy are distributed, copied, etc., and then often scanned back into digital form. This is referred to as a closed-loop process. Authenticating a scanned digital document refers to determining whether the scanned document is an authentic copy of the original digital document, i.e., whether the document has been altered while it was in the hard copy form. Alteration may occur due to deliberate effort or accidental events. There are two approaches to authenticating a printed document. The first approach utilizes a database that stores original document images, and compares the scanned document image with the original image.
The second approach eliminates the dependency on a database of original images. In particular, methods have been developed to authenticate a printed document using two-dimensional (2d) barcode. Typically, such a method encodes the content of the original document, or other information extracted from the original document that can be used to authenticate the document (generally referred to as authentication information), in 2d barcode (referred to as authentication barcode). The barcode is printed on the same recording medium as the printed document, e.g., on the front or back side of the printed document. The content of the document may be a bitmap image of a page of the document, text, graphics or images contained within the document, or a mixture thereof. To authenticate a printed document bearing an authentication barcode, the document is scanned to obtain scanned data that represents the content of the document, e.g. a bitmap image, text extracted by using an optical character recognition (OCR) technology, etc. The authentication barcode is also scanned and the data contained therein (the authentication data) is extracted. The scanned data is then compared to the authentication data to determine if any part of the printed document has been altered since it was originally printed, i.e. whether the document is authentic. Some authentication technologies merely determine whether any alterations have occurred, some are able to determine what content has been altered and what the alterations are. A printed document bearing authentication barcode is said to be self-authenticating because no information other than what is on the printed document is required to authenticate its content.